This invention relates to mufflers and particularly to mufflers for motorcycles which include structure for fluid treatment in addition to structure for silencing. Catalytic converters for treating automotive exhaust gases are available in a variety of configurations. Although such devices are commonly provided as a separate unit in addition to the usual muffler, it is known to provide a combined muffler and catalytic converter unit as taught by U.S. Pat. No. 3,445,196, for example. Providing a catalytic converter for use on motorcycles has been very difficult for several reasons. There is very limited space available in the region of the engine exhaust manifold such that an attempt to locate a catalytic converter near the manifold, where it operates most effectively, could result in too much heat too close to the driver's body and exhaust pulses of such magnitude and frequency that it would be most difficult to protect the catalyst element from being damaged thereby. Because of space limitations, a desire to shield the driver and any rider from excessive heat, and esthetic considerations, it would seem desirable to mount a catalyst element within a muffler housing. However, many problems are presented. These include reduction of the space available for sound treatment, increased backpressure, damage to relatively fragile catalyst elements by the exhaust pulses which are especially severe, and a difference in the coefficients of thermal expansion between the muffler housing and the catalyst element.